Design a scalable notification system

Q: Design a scalable notification system

This is a System Design interview question from Airbnb for Software Engineer roles. View the full question and solution on PracHub.

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Question

Problem

Design a notification system for a company at Airbnb-like scale that can send notifications to different user types (e.g., customers/guests, hosts, employees) through multiple delivery channels:

Email
SMS
Push notifications
Social/Tweet-like channel (optional integration)

The system must support two delivery modes:

Single / Event-triggered notifications : triggered by a real-time event (e.g., booking confirmed → email to guest; check-in day → SMS with lock code).
Bulk / Batch notifications : triggered by a scheduled or ad-hoc campaign to a target group (e.g., promotional content to potential hosts).

Expectations

Describe an end-to-end design that covers:

Core components and data flow for both event-triggered and batch sends
How messages are authored (templates), personalized, and routed to channels
User preferences (opt-in/out), frequency limits, and compliance (unsubscribe)
Reliability: retries, deduplication, ordering (if needed), idempotency
Scalability: high throughput, fan-out, backpressure, and rate limiting
Observability: logging, metrics, tracing, and auditing

You may assume existing product services emit events (booking, check-in, campaign creation, etc.) and your system consumes them to deliver notifications.

Design a scalable notification system

Problem

Expectations

Solution

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